Two main objectives for transportation of personnel or cargo on water can be distinguished: economical transportation (heretofore slow) and fast transportation (heretofore not economically efficient). The types of watercraft in use may be summarized as (1) displacement vessels, (2) planing hulls, (3) hydrofoils, (4) submarines, (5) non-ships like hovercrafts or other airborne configurations. Usually, the above-listed vessels are mono-hulls, although in principle they all could be used in a twin-hull or multi-hull configuration.
The achievable speed of a displacement vessel is limited to a Froude number of 1.3 (sometimes also called Taylor coefficient), a number which is proportional to the square root of the waterline length of a vessel. The physical reason for this limitation is that the displacement vessel creates a disturbance with its bow, which turns--as does any disturbance in water--into a wave. If the displacement vessel tries to go faster than the so created wave, it would have to climb over this wave. Any attempt to do so requires addition of more power. However, as additional power is applied, most of the power contributes to increasing the amplitude of the created wave. The ship therefore tries to climb over an ever-increasing mountain of its own making. The harder it climbs, the higher the mountain grows. Therefore, a displacement vessel cannot go faster than the velocity of the surface wave it creates.
To overcome this barrier the hull design has to be changed. The planing hull is one such design which is capable of going faster than the surface wave that it creates. The planing hull is more akin to an airplane than to a ship; the difference is that the weight of a displacement vessel is supported by the buoyancy (static) forces, while the weight of a planing hull vessel is supported by the induced lift (dynamic) forces. The consequence is that these induced forces cause extraordinary power (fuel) consumption. The planing hull is therefore inherently not suitable for fuel efficient transportation. Similar arguments are true for hydrofoils and hovercrafts.
The submarine is not limited in speed by the surface wave of the water, as long as the submarine is at least 3 hull diameters below the surface. There are of course frictional and other resistance forces impeding the movement of a submarine, however such forces act also on any other watercraft. Non-military transportation of personnel or cargo by submarines was proposed as early as 1914 and cargo submarines were indeed used during that era, yet this mode of transportation did not gain wide acceptance. The reason for this failure can be labeled the "Volume Problem" for the purposes of the present discussion. If one were to transport bulk cargo having a specific gravity larger than unity, an argument could indeed be made for the feasibility of a cargo submarine. However for personnel transportation, where there must be sufficient space for the occupants to move around, the Volume Problem exists. This means the void space provided for moving about will result in buoyancy that needs to be compensated for by ballast. For a submarine, which needs to be capable of surfacing, this ballast is usually water. Once inside the ballast tanks, this water is dead weight and needs to be transported as, in effect, useless cargo. Transportation of weight in any vehicle causes fuel consumption. Consequently, a vehicle transporting personnel underwater will consume more fuel underwater than a displacement vessel on top of the water carrying the same payload, provided the speed required is less than the maximum hull speed of the surface vessel.
Consequently, it is tempting to combine the advantages of the surface vessel with the advantages of the submarine, the main goal being to avoid the hull speed limitation of the surface vessel. Such proposals have been made in the past. U.S. Pat. No. 3,897,744, issued to Thomas G. Lang, is one example. Lang discloses two elongated hulls that are totally submerged and that support the ship above the water line. Ballasting chambers are disclosed as a part of these hulls; therefore this design suffers from the Volume Problem. Additionally, it does not eliminate the hull speed limitation. The reason that this is true is that for large ships the connection between the underwater hulls and the cargo hull have to be able to carry a substantial load. Long teaches that the buoyancy increases as the connections are further submerged. In this respect they act by displacement of water. These voluminous connections are also necessary to avoid the "Stability Problem." Therefore, these connections will have a substantial volume, which creates a surface wave, and this introduces a hull speed limitation. Ironically, the hull speed limitation caused by the connections is worse than the surface vessel would have since the waterline length of the connections (which determines the hull speed) is shorter than the waterline length the surface ship would have, and consequently the hull speed of the connection is slower than the hull speed of the surface ship. Also, these connections add additional weight and cost compared to a displacement vessel. The problem pointed out here is called, for the purposes of the present description, the "Connection Problem." This problem is inherent to the concept of underwater supporting hulls and therefore not restricted to the above cited patent.
There is still another problem inherent to the concept of combining the advantages of the surface vessel with the advantages of the submarine--the fact that the center of gravity is located above the center of buoyancy, which is an unstable configuration. As soon as they are no longer exactly vertically aligned, the center of gravity will move downward while the center of buoyancy will move upwards. The consequences will be that one of the submarine hulls will move to the surface, causing the surface platform to list. As the surface platform lists, the vessel's center of gravity moves from an original position which was between the two hulls toward a position above the submerged hull. Once the center of gravity passes the vertical above the submerged hull, buoyancy of the submerged hull will cause it to move upwards while the vessel's center of gravity will continue to move downward, thereby capsizing the vessel. For the purposes of the present description this problem is called the "Stability Problem."